Thank you for writing in. It is the policy of RubyGems.org that we refrain from making judgements about the content of a gem, as we have little to no insight into what development a user might be doing out of the public eye, or what their eventual plans are. We know this can be frustrating, especially in the case of popular or common names, but we would rather err on the side of protecting creators than accidentally removing someone’s work.

If you haven't already, I would attempt to contact the author and see if they'll add you as an owner of the gem. Instructions on how they can do that can be found here:

I was investigating a bug bounty program when I found an unclaimed gem, so
I squatted this name in order to report this to the company.
However upon re-investigating I now have noticed that this gem does not
belong to the bug bounty program, it was used as a library rather than a
self developed gem.

I will release this gem name by adding you as owner and remove myself as
owner of the gem.
Could you give me an email address so I can transfer the gem name?

As the maintainer of the https://github.com/prometheus/client_model repo, I would actually prefer not to have this gem at all (or at least I don't want to maintain it). The usage of the Prometheus protobufs in Ruby is almost non existing at the moment, and there are no plans to revive it. In fact, the upcoming standard for metrics exposition is OpenMetrics, which might then also provide a platform for protobuf support again, but that would be separate from the https://github.com/prometheus/client_model repo.